There are known practices for the anodization of surfaces of aluminum and aluminum alloy articles. Depending upon the intended use of the article it may be anodized for corrosion resistance, wear resistance and/or appearance. The surface of the article is cleaned and pretreated in preparation for anodization. There are many anodization practices, but in general the article is immersed as an anode in an aqueous electrolyte comprising an acid such as sulfuric acid. Anodization processes for producing colored or colorable anodized layers are often conducted with an electrolyte bath with a temperature of about 25° C. The passage of an electrical current, usually direct current, through the bath produces an adherent coating on the aluminum surface of closely spaced, crystalline columns of aluminum oxide. The resulting columnar coating is a result of competing chemical reactions between the electrolyte and the aluminum surface. The electrochemical effect is to oxidize aluminum atoms at the workpiece surface to Al2O3 which build-up on the surface as thin polygonal columns of the oxide with a vertical axial pore. Coincident with the oxidation of the aluminum surface atoms, the acid electrolyte tends to soften and dissolve the aluminum oxide columns. An exemplary thickness of the anodized oxide columns is about fifteen micrometers.
In some instances the composition of the aluminum alloy is such that the anodized layer is colored as formed. Some aluminum alloys, such as certain magnesium containing alloys, yield anodized layers that are grey or even black when anodized in a sulfuric acid electrolyte. Other aluminum alloys yield different colors. Sheets of these inherently colored, anodized aluminum alloys are useful for decorative purposes in architectural and building applications. Still other aluminum alloys yield clear anodized coatings that can be colored by dying or by electrolytic deposition of metal particles.
Recently, automotive vehicle body panels have been formed of sheet metal aluminum alloys and they have been painted to match other body surfaces. However, there is an interest in anodizing and coloring such aluminum alloy surfaces. But automotive outer surfaces are continually exposed to the color degrading effects of the ultraviolet portion of solar radiation. Most dyes used in coloring anodized surface quickly fade during prolonged exposure to sunlight and are unsuitable for vehicular external applications. The coloring effect of metal particles deposited in the pores of the alumina columns is more durable than typical dyes but still fades to an unacceptable degree.
Accordingly, it is an object of this invention to provide a method of stabilizing the color of anodized aluminum coating against degradation by ultraviolet radiation.